The present invention relates to a method of controlling the combustion process in the combustion chamber in a four-stroke internal combustion engine with at least one cylinder having at least one inlet valve and at least one exhaust valve, comprising supply of a homogeneous fuel/air mixture to the combustion chamber and compression of the mixture to self-ignition.
The invention also relates to a four-stroke internal combustion engine with at least one cylinder having at least one inlet valve and one exhaust valve, means for supplying a homogeneous fuel/air mixture to the cylinder combustion chamber and control means for at least varying the degree of opening of the exhaust valve.
In compression ignition of a homogeneous fuel/air mixture, so-called HCCI (Homogeneous Charge Compression Ignition) in a four-stroke internal combustion engine, a homogeneous diluted (with extra air or residual gas) fuel/air mixture is compressed to self-ignition. The advantage of this compared to first compressing the inlet air and then injecting fuel into the combustion chamber (the diesel process) is that the entire fuel/air mixture burns simultaneously and not successively as when a flame front propagates through the combustion chamber from a sparkplug or injector. This creates a homogeneous temperature in the combustion chamber, which in turn makes it possible to achieve, for example in an unthrottled Otto-engine at partial load, the efficiency of the diesel engine but without the high nitrogen oxide and particle emissions of the diesel engine. The nitrogen emissions can be reduced from ca. 1000 ppm to as little as 10-20 ppm. The particle emissions of the diesel engine can be reduced to the same level as those of the Otto-engine. The difficulty is, however, to control the combustion since it is kinetically controlled. If the mixture is too rich, the energy released will be too rapid (knocking), and if it is too lean, ignition will be made impossible. In an HCCI Otto-engine with gasoline as fuel, a high and controlled temperature is required to achieve self-ignition, and this can be achieved with high compression ratio and/or by heating the inlet air. In an HCCI diesel engine with diesel oil as fuel, lower temperatures are required than with a normal diesel engine, which means that the compression ratio must be lowered.
The difficulty up to now in HCCI engines has been to control the ignition delay (the cylinder temperature) in such a manner that the combustion is positioned correctly about the top dead center at various rpms and loads, and this has greatly reduced the range of use of such engines. Especially, control problems during transients, where the cylinder temperature must be checked from one cycle to the next, have limited the range of use of HCCI engines to generators, for example, where the drive unit operates with very small variations in rpm and load.
The purpose of the present invention is to achieve a method of controlling the temperature in the cylinders in an HCCI engine, so that the ignition time will be correct at various engine speeds and loads, thereby making it practically possible to use HCCI engines in motor vehicles, thereby reducing their fuel consumption and emissions.
This is achieved according to the invention by virtue of the fact that the exhaust valve during the exhaust stroke of the piston is controlled so that it, at least within a lower rpm range, closes before the piston has reached its upper dead center point, the degree of opening of the valve being varied, depending on engine load and rpm so that the amount of residual gas at closing of the valve is greater at low loads and high rpm than at high loads and low rpm, to thereby vary the ignition delay by varying the degree of dilution of the fuel/air mixture with residual gases in the combustion chamber.
Complete freedom of valve control, so that the opening and closing times can also be freely controlled from cycle to cycle, can be achieved by using electromagnetically operated valves. The amount of residual gases which determine the temperature in the combustion chamber and thus the ignition time, can be regulated in this manner within a broad interval, within which the temperature increases the earlier the exhaust valve is closed.
In order to prevent increase in engine pump work (pump losses), according to a further development of the method according to the invention, the inlet valve is so controlled during the inlet stroke of the piston, that, at least within said lower rpm range, it opens a certain number of crankshaft degrees after the upper dead center point of the piston, the degree of opening being varied relative to the degree of opening of the exhaust valve, to reduce residual gas pressure in the combustion chamber to the inlet air pressure.
An HCCI engine which must be able to operate within a wide rpm range, e.g. with an upper limit of about 6000 rpm, is preferably equipped with an ignition system which is controlled so that it is deactivated within said lower rpm range, the upper limit of which can lie between 3000 and 4000 rpm. When this limit is exceeded, the ignition system is activated at the same time as the control of the exhaust and inlet valves is changed to normal engine operation, i.e. the exhaust valve is controlled so that it closes after the piston, during the exhaust stroke, has reached its upper dead center, at the same time as the inlet valve is controlled so that it begins to open before the exhaust valve is completely closed. The same applies at high load and low rpm, since otherwise the rate of combustion will be too high as the degree of dilution is reduced.
An internal combustion engine of the type described by way of introduction, which is to be controlled in the above described manner, is characterized in that the control means are so arranged to control the exhaust valve during the exhaust stroke of the piston, that the valve, at least within a lower rpm range, closes before the piston has reached its upper dead center point, and that the degree of opening of the valve is varied, depending on the engine load and rpm, so that the amount of residual gases in the combustion chamber at the closing of the valve is greater at lower loads and higher rpm than at higher loads and lower rpm.